On Sunday, April 13 (103/22:21:08 GMT) HST entered Zero Gyro Sun Point Safemode due to system momentum failing in response to a stalled gyro 3. The current theory of this malfunction is that a gathering of lubrication material results in transient load torques causing the motor to draw more current before it finally stalls. The anomaly is sensed by the flight software, which commands the vehicle into the special safemode in which the SI C&DH and some Instruments are safed and the aperture door is closed resulting in an interrupt of science operations. Once the situation was understood, recovery from the safemode was initiated by a Health and Safety SMS with a final intercept of the normal Science SMS on Monday, April 14 2003 ( 105/02:00 GMT).
Gyro 3 anomaly
The Rate Gyro Assembly ( RGA) is part of the many sensors feeding the primary control law to manage the vehicle dynamics. For science 3 gyros are routinely in the control loop.
Fig 1: Rate Gyro Assembly (RGA)
The gyro 3 motor current began to increase at 103/22:18:30 GMT and stabilized at 330 mA approximately 90 seconds later. The motor current reached 347 mA just prior to stabilizing at 330 mA. All indications including the motor current profile appear to be similar to the gyro 3 event on Thanksgiving Day (332/2002) in 2002.
Fig 2: gyro 3 motor current profile before stalling
It is standard procedure to restart a stalled gyro since the lubrication material that caused the stall moved by the spin down and spin up process. At 103/23:51:28 GMT gyro 3 was command on and after the expected turn-on spike saddled to a reasonable motor current at a nominal level. The motor current stabilized at approximately 196 mA varying by +/-4 mA (2 raw counts).
Fig 3: gyro 3 motor current profile before during power up phase
During the Zero Gyro Sun Point Safemode all subsystems reported nominal performance. ACS, STIS, and COSTAR have been safed due to the vehicle safing event. Recovery of the instruments progressed with following standard procedures and with successfully completed prior the resume science on Monday, April 14 2003 (105/02:00 GMT).
Report written by Manfred Miebach